This invention relates generally to step-and-repeat alignment and exposure systems for aligning each of an array of different regions of an object, such as a semiconductive wafer, with respect to an image of another object, such as a reticle, and for photometrically printing that image at each of those regions by employing a projection lens of the reduction type. More specifically, this invention relates to improved and simplified optical apparatus for use in such systems to facilitate utilization of the projection lens in aligning each region of the semiconductive wafer with respect to the image of the reticle.
In U.S. patent application Ser. No. 396,099 an improved step-and-repeat alignment and exposure system is disclosed that may be employed for aligning each of an array of inchoate dice regions of a semiconductive wafer with respect to an image of a main reticle containing a level of microcircuitry to be photometrically printed at each of those inchoate dice regions. The system includes a main stage movable along orthogonal axes of motion of the system, a substage mounted on the main stage for aligning a reference mark with respect to the orthogonal axes of motion of the system, a rotatable vacuum chuck mounted on the main stage for holding the semiconductive wafer, a reticle stage mounted above the main stage for holding the main reticle, and a projection lens disposed between the main stage and the reticle stage and operable with an associated beam splitter and an associated split-field objective lens unit for directly viewing in aerial image of the image of tne main reticle and also of the reference mark or the semiconductive wafer depending on the position of the main stage.
Thus, while employing the projection lens and its associated split-field objective lens unit to directly view the aerial image of the image of the main reticle and also of the reference mark, the main stage and the substage can be employed to align the reference mark with respect to the orthogonal axes of motion of the system during a reference mark set-up alignment operation, and the main stage and the reticle stage can thereafter be employed to align images of global alignment marks disposed at opposite sides of the main reticle with respect to corresponding portions of the reference mark and, hence, with respect to the orthogonal axes of motion of the system during a main reticle alignment operation. Similarly, while subsequently employing the projection lens and its associated split-field objective lens unit to directly view the image of the image of the main reticle and also of the semiconductive wafer, the main stage can be employed to alternately align global alignment marks disposed at opposite sides of the semiconductive wafer with respect to the images of the global alignment marks of the main reticle to align the semiconductive wafer as a whole with respect to the orthogonal axes of motion of the system during a global wafer alignment operation, and thereafter to align local alignment marks disposed between selected ones of the inchoate dice regions of the semiconductive wafer with respect to an image of a corresponding local alignment mark on the main reticle to more precisely align each inchoate dice region with respect to the image of the main reticle during a precision region-by-region or local wafer alignment operation.
The projection lens is corrected for blue exposure light having a wavelength of 436 nanometers so that the projection lens can be employed to photometrically print the level of microcircuitry contained on the main reticle at each inchoate dice region of the semiconductive wafer, once that region is aligned with respect to the image of the main reticle, in precise alignment with other previously or yet-to-be printed levels of microcircuitry by exposing a photoresistive film on that region in accordance with that image during a wafer exposure operation. A controllable light source unit, an associated plurality of masking elements, another beam splitter and an imaging lens disposed between that beam splitter and the main reticle are provided for selectively illuminating different portions of the main reticle with exposure light so that the global and local alignment marks of the semiconductive wafer can be illuminated with exposure light during the global and precision local wafer alignment operations performed with the projection lens and its associated split-field objective lens unit without exposing the photoresistive film on each inchoate dice region. However, due to interference patterns that may be created for some photoresistive films or other surface conditions of the semiconductive wafer at the wavelength of the exposure light for which the projection lens is corrected, it is often desirable to employ a photoresistive film that is opaque to the exposure light. In this case the projection lens and its associated split-field objective lens unit cannot be employed in performing the global and precision local wafer alignment operations.
The controllable light source unit can be adjusted to selectively illuminate the reticle with green nonexposure light having a wavelength of 546 nanometers, and a compensating lens can be moved into position to compensate the projection lens for the green nonexposure light during global and precision local wafer alignment operations involving a photoresistive film that is opaque to the exposure light. However, a serious disadvantage of this technique is the loss of time in adjusting the controllable light source unit and moving the compensating lens into position for the global and precision local wafer alignment operations and thereafter readjusting the controllable light source unit and moving the compensating lens back out of position for the wafer exposure operation. Moreover, the image plane of the projection lens may be shifted by the compensating lens, thereby resulting in the possibility of serious alignment errors.
In U.S. patent application Ser. No. 389,678 a bore-sighted step-and-repeat alignment system is disclosed in which the projection lens may be employed without a compensating lens in performing global and precision local wafer alignment operations involving a photoresistive film that is opaque to the exposure light. This system also employs a split-field objective lens unit, operable with the beam splitter, the projection lens and a source of exposure light (employed in place of the controllable light source unit), for directly viewing aerial images of images of the global alignment marks of the main reticle and also of the corresponding portions of the reference mark to facilitate alignment of the images of those global alignment marks with respect to the reference mark during the main reticle alignment operation. A first single-channel objective lens unit, operable with a chromatic beam splitter, a nonchromatic beam splitter, the projection lens and the source of exposure light, is employed for directly viewing an aerial image of an image of another alignment mark disposed on the main reticle between the global alignment marks and also of a corresponding portion of the reference mark to facilitate alignment of that corresponding portion of the reference mark with respect to the image of the other alignment mark. In addition, a second single-channel objective lens unit, operable with both the chromatic and nonchromatic beam splitters, the projection lens and a first source of yellow nonexposure light having a wavelength of 577 nanometers, is employed for imaging an alignment mark of an adjustable alignment reticle at the image plane of the projection lens and for directly viewing an aerial image of the image of that alignment mark and also of a corresponding portion of the reference mark. This facilitates adjustment of the alignment reticle to align the image of its alignment mark with respect to the corresponding portion of the reference mark and, hence, with respect to the image of the other alignment mark of the main reticle in a bore-sighting alignment reticle set-up alignment operation. The bore-sighting alignment reticle set-up alignment operation can be repeated as frequently as desired during the operation of the bore-sighted step-and-repeat alignment and exposure system to maintain the alignment accuracy of the system.
Following the bore-sighting alignment reticle set-up alignment operation, the second single-channel objective lens unit can be employed for directly viewing an aerial image of the image of the alignment mark of the alignment reticle and also of each global alignment and local alignment mark of the semiconductive wafer to facilitate alignment of each of those alignment marks of the semiconductive wafer with respect to the image of the alignment mark of the alignment reticle during the global and precision local wafer alignment operations. Additional optical apparatus, including an apertured field-stop plate and a second source of yellow nonexposure light, is employed for illuminating the entire field viewed in the image plane of the projection lens by the second single-channel objective lens unit with the yellow nonexposure light to facilitate locating the global alignment marks of the semiconductive wafer during the global wafer alignment operation.
A disadvantage of both of the foregoing step-and-repeat projection alignment and exposure systems is that the nonchromatic beam splitter associated with each split-field objective lens unit is not a commercially available off-the-shelf part. Concomittantly, neither the projection lens nor either split-field objective lens unit is a commercially available off-the-shelf part. Moreover, the optical portions of both systems are relatively complex, and different light sources are employed for alignment and exposure operations resulting in possible misalignments due to different intensity distributions of those light sources.
Accordingly, it is the principal object of this invention to provide an improved step-and-repeat projection alignment and exposure system in which the need for the aforementioned nonchromatic beam splitter is eliminated and in which a commercially available off-the-shelf projection lens and a commercially available off-the-shelf split-field objective lens unit can be employed.
Another object of this invention is to provide a step-and-repeat projection alignment and exposure system, such as either of the aforementioned systems, with improved and simplified optical apparatus for use in performing the aforementioned reticle and wafer alignment operations.
Another object of this invention is to provide an improved step-and-repeat projection alignment and exposure system in which one light source is employed for both alignment and exposure operations, thereby eliminating possible misalignments due to different intensity distributions of different light sources.
Still another object of this invention is to provide an improved and simplified bore-sighted step-and-repeat projection alignment and exposure system.
These and other object, which will become apparent from a reading of this specification and an inspection of the accompanying drawings, are accomplished according to the illustrated preferred embodiment of this invention by employing a source of exposure light including a shutter with a neutral density filter operable, when the shutter is closed, for passing enough exposure light to illuminate the main reticle without exposing the photo-resistive film on the semiconductive wafer; by replacing the nonclaromatic beam splitter disposed between the main reticle and the projection lens with a cold mirror disposed for reflecting exposure light from the source of exposure light and for transmitting nonexposure light from first and second sources of nonexposure light; by replacing the split-field objective lens unit operable with the replaced nonchromatic beam splitter with a split-field objective lens unit operable with the other beam splitter and the imaging lens, mentioned above in connection with the system disclosed in U.S. patent application Ser. No. 396,099, for directly viewing an aerial image of illuminated portions of the main reticle and of the reference mark or the semiconductive wafer; and by employing a silicon intensified TV camera and an associated TV monitor for displaying that image.